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Models in Geometry and Logic: 1870-1920 ∗ Models in Geometry and Logic: 1870-1920 ∗ Patricia Blanchette University of Notre Dame 1 Abstract Questions concerning the consistency of theories, and of the independence of axioms and theorems one from another, have been at the heart of logical investigation since the beginning of logic. It is well known that our own contemporary methods for proving independence and consistency owe a good deal to developments in geometry in the middle of the nineteenth century, and especially to the role of models in estab- lishing the consistency of non-Euclidean geometries. What is less well understood, and is the topic of this essay, is the extent to which the concepts of consistency and of independence that we take for granted today, and for which we have clear techniques of proof, have been shaped in the intervening years by the gradual de- velopment of those techniques. It is argued here that this technical development has brought about significant conceptual change, and that the kinds of consistency- and independence-questions we can decisively answer today are not those that first motivated metatheoretical work. The purpose of this investigation is to help clarify what it is, and importantly what it is not, that we can claim to have shown with a modern demonstration of consistency or independence.1 ∗This is the nearly-final version of the paper whose official version appears in Logic, Methodology, and Philosophy of Science - proceedings of the 15th International Congress, edited by Niiniluoto, Sepp¨al¨a,Sober; College Publications 2017, pp 41-61 1Many thanks to the organizers of CLMPS 2015 in Helsinki, and also to audience members for helpful comments. This essay includes material previously presented at Logica 2015 in Hejnice, Czech Republic and published in Logica 2015, as well as material presented earlier at the Ohio State University and at the University of Notre Dame; thanks also to all three audiences and to 1 2 Introduction In 1939, Ernest Nagel wrote: The sort of questions considered by meta-mathematics, e.g. the consis- tency and independence of axioms, have been discussed in antiquity and have been cultivated ever since by mathematicians in every age; but such problems have been clearly formulated and systematically explored only after pure geometry had been freed from its traditional associations with space, and only after its character as a calculus had been isolated from its applications. ([Nagel 1939] x74, pp 202-3) While there is no doubt a good deal of truth to this claim, the purpose of this paper is to suggest a way in which the characterization it gives us of the role of models in meta-mathematics is misleading. The aspect of Nagel's view that I want to question is the idea that with the increasing formalization of mathematical theories in the late 19th and early 20th centuries, we achieved at last a method, and tools, for the rigorous treatment of ancient questions. What I will suggest in what follows is that the questions we can now answer with our modern rigorous tools are not the same questions as those that arise for mathematical theories prior to the modern era. The questions we can now raise and answer, I will argue, have been shaped significantly by the tools we have developed. One result of this is that the notions of independence and consistency that we now take for granted are not those that went by these names prior to about 1900, and the clean methods we now have for demonstrating independence and consistency do not answer what in e.g. 1700, or even in 1850, would have been called by these names.2 3 1900 We begin by looking at the state of the art of independence proofs in the penul- timate year of the nineteenth century. Here our example is David Hilbert's 1899 Foundations of Geometry, in which we find a clear and systematic application of the technique that was becoming, at this point, the standard approach to demonstra- the Logica 2015 organizers. 2On the role of geometry in the early development of modern logic, see [Webb 1985]. 2 tions of independence.3 As Felix Klein puts it in 1908, describing what he calls the \modern theory" of geometric axioms: In it, we determine what parts of geometry can be set up without using certain axioms ... As the most important work belonging here, I should mention Hilbert's [1899]. ([Klein 1908], as quoted in [Birkhoff and Bennett 1988] p 185.) Hilbert's work in the Foundations of Geometry monograph involves a clear and care- ful axiomatization of Euclidean geometry, together with a consistency proof for the whole, and a series of independence proofs that demonstrate the connections of logi- cal entailment holding between the various parts of the edifice. Hilbert characterizes both consistency and independence here in terms of the relation of logical deducibil- ity: a set of axioms is consistent, he tells us, if \[I]t is impossible to deduce from them by logical inference a result that contradicts one of them" [x9], and a geometric axiom or theorem is independent of a collection thereof if it cannot be deduced from that collection. HIlbert's method of demonstrating non-deducibility is as follows: Given a set AX of axioms, and a statement (perhaps another axiom) A, we begin by uniformly re- interpreting the geometric terms (\point," \line," \lies-on," etc.) in AX and in A, in terms of objects and relations given by a different theory, in this case a theory R of real numbers and collections thereof. We then note that, as re-interpreted, each of the sentences AX, together with any sentence deducible from them, expresses a theorem of R. Finally, the negation :A of the target sentence A also expresses a theorem of R, which, assuming the consistency of R, guarantees that A itself does not express a theorem of R. Still assuming the consistency of R, then, we have a guarantee that A is not deducible from AX. The consistency of AX, in the sense of the non-deducibility of a contradiction from it, is demonstrable similarly, again assuming the consistency of the background theory R. As Hilbert says, From these considerations, it follows that every contradiction resulting from our system of axioms must also appear in the arithmetic related to the domain [of the background theory].([Hilbert 1899] x9) 3See [Hilbert 1899]. Hilbert was not alone in using the technique to significant effect at this time. Its application can be seen as well in the work of the Italian school (see e.g. [Peano 1894], [Padoa 1901], [Pieri 1898]; for discussion of Pieri's work see [Marchisotto and Smith 2007]) and in closely-related work by e.g. Veblen [Veblen 1904] and Dedekind [Dedekind 1888]. 3 An important point to note about the interpretation-theoretic technique used by Hilbert here is that it presupposes that the relation of deducibility in question is \formal" in the sense that it is unaffected by the reinterpretation of geometric terms. It is this that guarantees that the sentences deducible from AX will express theorems of R under the reinterpretation, given only that the members of AX do. But the deducibility relation is not, for Hilbert in 1899, \formal" in the sense of \syntactically specified;” there is no formal language at this point, and no explicit specification of logical principles. We will use the term \semi-formal" for such a relation. The first thing, then, that Hilbert's interpretations (or models) shows is that a given sentence is not semi-formally deducible from a collection of sentences. Hilbert's models also show, importantly, the satisfiability of the conditions implicitly defined by the collections of sentences in question. Given a collection AX [:A of sentences whose geometric terms appear schematically, a Hilbert-style reinterpreta- tion on which each member of that collection expresses a truth about constructions on the real numbers demonstrates the satisfiability of the condition defined by the collection. Equivalently, it demonstrates that the condition defined by AX can be satisfied without satisfying the condition defined by A.4 Hilbert's technique, then, demonstrates the independence of a given sentence from a collection of sentences in two different senses. Taking as an example the question of the independence of Euclid's parallel postulate (PP) from the remainder of the Euclidean axioms (EU) for the plane, the two senses, with our labels introduced, are: • IndependenceD: (PP) is not (semi-formally) deducible from (EU); • IndependenceS: The condition defined by (EU) [:(PP ) is satisfiable. IndependenceS is the stronger of the two notions, though they are extensionally equivalent in the setting of an ordinary first-order language.5 4This understanding of consistency in terms of satisfiability was the more central concern for e.g. Dedekind and Veblen 5The equivalence is given by the completeness of first-order logic. Hilbert's setting is that of natural language without strictly-defined relations of deducibility or satisfiability, so the question of the extensional relationship between the two independence relations is imprecise. The expressive richness of that language, however, is well beyond that of (what was to become) first-order logic, giving the second relation, in that setting, a narrower extension than the first. 4 4 Frege Gottlob Frege's work, in the same period, focuses on a notion of independence that's distinct from both of the relations demonstrable via Hilbert's technique. For Frege, independence is a relation not between sentences but between thoughts, i.e. be- tween the kinds of things expressible by fully-interpreted sentences. Each thought, as Frege understands it, has a determinate subject-matter: thoughts about geomet- ric objects and relations are entirely distinct from thoughts about collections of real numbers.
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